The present invention relates to a device for preventing prolonged depression-action type activation in a contact mechanism, and more particularly, concerns a push-button switching device which permits electrical contact to be broken even while an operator of the device holds the push-button down so that the switch is in the normally closed position.
Various types of switches and contact mechanisms operable by depression-action activation are employed particularly for heavy duty, industrial-type situations for making an electrical connection to a power source. Panel mounted, wall mounted, or switch box push-button type switches are most prevalent for these switching type needs. One type of push-button switch is the momentary type wherein electrical contact is established by depressing the button inwardly; as long as the operator holds the button down, electrical contact is maintained. As soon as the button is released, electrical contact is broken and the switch is open, thereby de-energizing the system. Such a momentary switch thus requires the operator's attention to assure that electrical contact is made only for the necessary periods of time. Another type of push-button switch is one which maintains contact. This type of switch includes a button which remains depressed upon the initial downward movement to close the switch; to open the switch and break electrical contact, the operator must depress the button inward slightly in order to release the spring mechanism and thereby return the button to the original starting position. Each of these previously described switches serves the needs for specific switching applications effectively, but, on the other hand, have a number of deficiencies.
For instance, when using the momentary switch for establishing an electrical connection, the system is generally wired so that only a brief depression of the button is necessary to close the switch and energize the system. This brief period of time may be as short as one second; thus, the operator to activate the system merely depresses the button quickly, then releases same in order to establish electrical contact. The initial depression generally signals the switch to close and may include one or more electrical relays or the like in order to maintain the electrical contact upon the initial, brief depression of the button. When electrical contact is to be broken, another brief depression of the button opens the switch by means of appropriate circuitry including relays and the like. It can then be appreciated that, with the switching circuitry involved, prolonged depression of this type of push-button switch may induce problems. Specifically, if the operator seeks to assure that electrical contact is established and holds the button downwardly too long, it is conceivable that a sensitive electrical system may be overloaded, with some type of power failure in the system then being induced. In other words, with those type of push-button switches which are meant to establish electrical contact with only a brief, firm depression of the button, problems may arise when the button is held down unnecessarily lengthy periods of time. This is indeed a problem inasmuch as the operator may not even be aware that holding this type of push-button switch down too long could cause a problem; thus, the operator oftentimes attempts to push the button down for a sufficiently long period of time to assure electrical contact without really knowing that he may be inducing some damage or failure.
An anti-holddown switch actuating mechanism which will open after a predetermined time after actuation, regardless of whether the operator maintains pressure on an operating knob, has been described in U.S. Pat. No. 3,843,855, issued to Danielson. In the Danielson disclosure, an operating knob, upon being downwardly depressed, causes the air pressure in a cylindrical bore to increase, which inturn distends a diaphragm downwardly. A contact button on the bottom of the diaphragm also moves downwardly, depressing a plunger to close the switch. The increase in air is bled off through a small orifice. Regulation of bleeding is controlled by a tapered portion of an adjustment screw. Air is allowed to re-enter the piston through a one-way fluid diode. While the Danielson patent indeed discloses one type of anti-holddown device, some problems are inherent in the structure and configuration proposed by Danielson. For instance, inasmuch as air starts to bleed as soon as the knob is pushed, if the operator pushes slowly (virtually at the same speed as the air bleed-off rate) the device will not function properly. Also, the strength of Danielson's diaphragm also contributes to the functionability of his device. Specifically, if the diaphragm is flimsy or weak, the spring force in the contact block would not be overcome, so that the plunger in the bottom block would never be depressed sufficient to close the switch. On the other hand, if the spring in the bottom contact block of Danielson is too weak, it will not be strong enough to push the diaphragm completely back to its initial starting position, thereby impairing its functionability. Furthermore, the working of Danielson's anti-holddown actuating mechanism relies upon the spring element in the regular bottom contact block, which is found in many standard industrial type switches. Thus, the adaptability of Danielson's device is limited to those instances where a spring loaded plunger is provided to return the contact button on the diaphragm back to its starting position. Thus, it can be seen that further improvements in this type of device are still being sought, and it is to this end that the present invention is directed.